IP Address to Hostname Resolution Issues

You see a '?' symbol in the hostname column next to the IP address

The '?' symbol means that the IPv4 address was not 'resolved' to a hostname.

Why? usually it means that the DNS your computer communicates with does not have PTR record indicating a hostname for the IPv4 address or the DNS did not respond (not operational or wrong IP) or the left control panel checkbox 'Resolve IPs to Hostnames' is unchecked.

When we talk about 'IP Addresses' below we are referring to the column with the heading IP Address. When we talk about Hostnames, we are referring to the column with the heading 'Hostname from IP'.

Problems and causes. Review this before contacting support:

• Do you see any IPv4 addresses matching the MAC addresses? if the IP Address column is empty, then we cannot find the hostnames. Your problem is that your ARP tables used to find IPv4 addresses given a MAC address are not suitable.
• You are seeing IPv4 addresses and you seeing no hostnames at all: Make sure you have the checkbox on the left control panel 'Resolve IPs to Hostnames' checked. If it is checked your DNS may not be working or you may have the wrong IPv4 address for the DNS or the DNS has no PTR records for the IPv4 addresses.
• You are seeing some hostnames matching the IPv4 addresses: this means our IP to Hostname resolution method is working and your DNS does not have PTR records for each IPv4 address.
• You are seeing hostnames but some hostnames are wrong matching the IPv4 addresses: this means our IP to Hostname resolution method is working and your DNS does not have up-to-date PTR records for each IPv4 address.

DNS Settings

You need to verify that the DNS IPv4 addresses are correct. Do this by clicking on the left control panel button Global Settings. Under the grouping IP to Hostname resolver have a look at these selections:

• Default System DNS - when selected, are the DNS IPs correct? (we are showing the order they are searched by the OS - we have no control over this order - this is a read only field)
• User Defined DNS - when selected, is it the DNS IPv4 address correct? (you can enter a DNS IPv4 address here)
• No DNS - when selected, it means 'Resolve IPs to Hostnames' is not checked and IPs will not be resolved to hostnames. (the resolver function is OFF)

Background: IPv4 addresses are resolved using a PTR record request to DNS using the IP address as input. You must have a valid DNS assigned by DHCP or statically assigned - check your DNS with command line 'ipconfig -a'. The DNS needs to have a PTR record for the IPv4 address or you will get nothing.

Observation: We have found many internal company DNS do not have valid or up-to-date PTR records and yet they do have a valid A record (used for hostname to IPv4 address queries). If there is no PTR record for the IP address, you will not see a hostname. (We do not use A records.)

Here is the process use by the operating system in 'Default System DNS Mode' to obtain the hostname for the IPv4 address.
The IP address is passed to the operating system's 'resolver' which is a standard Windows Win32 function called 'gethostbyaddr'. gethostbyaddr (resolver function) works by first looking at your hosts file (C:\Windows\System32\drivers\etc\hosts), then failing that, the function sends a PTR query to each DNS in order (as configured in your system by DHCP or static network interface settings). If each DNS fails to return an answer the function does a NetBIOS Name Server Node Status Request query from port 137 of your machine to port 137 on the target machine (via UDP). If successful, the NetBIOS query returns the windows computer name. If gethostbyaddr fails, we place a '?' in the hostname column. Test NetBIOS this way: Use command line nbtstat -a ipAddress to test Windows target to try to see NetBIOS computer name. How to enabled NetBIOS over TCP/IP - locate the network connection, right click and select Properties. The click to highlight Internet Protocol Version 4 (TCP/IPv4) and press the Properties button. Press the Advanced button on the General tab. Select the WINS tab and Enable NetBIOS over TCP/IP. Then press OK twice to exit then press Close.

The process used when 'User Defined DNS' is selected is simpler.
The IPv4 address is passed to a custom resolver which does a PTR record request to the DNS you specified. Only one DNS can be specified and if it fails to return a response or gives a 'not found' response, we place a '?' in the hostname column.

Contact Support

If you need more help, contact support here.